What is the cost function of a transformer?
I took a look at the Tensor2Tensor's source code implementation as per @nbro's suggestion, and it seems like the loss function is the cross entropy between the predicted ($\|sentence length\|$ x $\|vocab\|$) probability matrix (right before taking the argmax to find the token to output), and the $\|sentence length\|$-length vector of token IDs as the true label.